As an exhaust gas after-treatment device configured to treat an exhaust gas emitted from an internal combustion engine, an exhaust gas after-treatment device having a NOx-occlusion-reduction-type catalyst and an exhaust gas after-treatment device having a NOx-occlusion-reduction-type catalyst and an oxidation catalyst have been known (for example, refer to Patent References 1 and 2). The NOx-occlusion-reduction-type catalyst reduces and purifies NOx (nitrogen compound) in the exhaust gas emitted from the internal combustion engine. The oxidation catalyst is arranged at an upstream-side of the NOx-occlusion-reduction-type catalyst and is supplied with unburnt fuel by post injection of an exhaust gas injector or an in-cylinder injector, thereby increasing an exhaust gas temperature.
When the exhaust gas is in a lean atmosphere, the NOx-occlusion-reduction-type catalyst occludes NOx contained in the exhaust gas, and when the exhaust gas is in a rich atmosphere, the NOx-occlusion-reduction-type catalyst detoxifies and releases the occluded NOx with hydrocarbon contained in the exhaust gas by reduction and purification. For this reason, in a case where a NOx occlusion amount of the catalyst reaches a predetermined amount, it is necessary to periodically perform NOx purge of enriching the exhaust gas by post injection or exhaust gas pipe injection so as to recover a NOx occlusion capacity (for example, refer to Patent Reference 1).
Also, sulfur oxide (hereinafter, referred to as ‘SOx’) contained in the exhaust gas is occluded in the NOx-occlusion-reduction-type catalyst. When a SOx occlusion amount increases, a NOx purification capacity of the NOx-occlusion-reduction-type catalyst is deteriorated. For this reason, in a case where the SOx occlusion amount reaches a predetermined amount, it is necessary to periodically perform SOx purge of supplying unburnt fuel to the upstream oxidation catalyst to increase an exhaust gas temperature to a SOx separation temperature by post injection or exhaust gas pipe injection so as to separate SOx from the NOx-occlusion-reduction-type catalyst for recovery from the sulfur poisoning (for example, refer to Patent Reference 2).